Ceramic porous membrane and ceramic filter

ABSTRACT

There are disclosed a ceramic porous membrane formed with less membrane formation times and having less defects, a small and uniform thickness and a high resolution, and a ceramic filter. A silica membrane as a ceramic porous membrane is formed on a titania UF membrane as an ultrafiltration membrane (a UF membrane) formed on a porous base member which is a microfiltration membrane (also referred to as an MF membrane) and having an average pore diameter smaller than that of the porous base member, the silica membrane has an average pore diameter smaller than that of the titania UF membrane and a part of the silica membrane permeates the titania UF membrane.

BACKGROUND OF THE INVENTION AND RELATED ART

The present invention relates to a ceramic porous membrane and a ceramicfilter. More particularly, it relates to a ceramic porous membranehaving less defects and having a small and uniform thickness, and aceramic filter.

Heretofore, various methods of forming a ceramic porous membrane on aporous base member have been known. For example, a hot coating processis known (see Non-Patent Document 1). This is a method of rubbing a tubebase with cloth containing a silica sol to apply the silica sol andthereby form a porous membrane on an outer surface of the heated tubebase.

A method of forming a porous membrane on an inner surface of a porousbase member having a tubular shape or a cylindrical lotus-root-likemonolith shape by filtering membrane formation is also known (see PatentDocuments 1, 2). The outer surface of the porous base member is held ata pressure lower than that of an inner surface thereof which comes incontact with a sol liquid to form the membrane on the inner surface ofthe porous base member.

-   [Patent Document 1] Japanese Patent Application Laid-Open No.    3-267129-   [Patent Document 2] Japanese Patent Application Laid-Open No.    61-238315-   [Non-Patent Document 1] Journal of Membrane Science 149 (1988) 127    to 135

However, the hot coating process has a problem that the membrane cannotuniformly be formed on the whole base surface, and the membrane can beformed on the only outer surface of the tube base. The process cannot beapplied to any monolith-type base. On the other hand, in the filteringmembrane formation process, during drying of the formed membrane, asolvent present in base pores sometimes flows out on a membrane side tocause membrane peeling. As a result, there is a problem that a defect isgenerated in the porous membrane formed on the fired base surface. A dipcoating process can be applied to the monolith type base, but the numberof membrane formation times is large.

An object of the present invention is to provide a ceramic porousmembrane formed with less membrane formation times and having lessdefects, a small and uniform thickness and a high resolution, and aceramic filter.

SUMMARY OF THE INVENTION

The present inventors have found that the above-mentioned object can beachieved using a constitution in which the ceramic porous membrane isformed on an ultrafiltration membrane so as to partially permeate poresof the ultrafiltration membrane. That is, according to the presentinvention, the following ceramic porous membrane and ceramic filter areprovided.

[1] A ceramic porous membrane which is formed on an ultrafiltrationmembrane having an average pore diameter of 2 to 20 nm and whichpartially permeates pores of the ultrafiltration membrane.

[2] The ceramic porous membrane according to the above [1], wherein apermeation depth in the ultrafiltration membrane is ½ or more of athickness of the ultrafiltration membrane from the outermost surface ofthe ultrafiltration membrane.

[3] The ceramic porous membrane according to the above [1] or [2],wherein the ultrafiltration membrane is a titania membrane.

[4] The ceramic porous membrane according to any one of the above [1] to[3], which is a silica membrane.

[5] The ceramic porous membrane according to any one of the above [1] to[4], wherein a ceramic sol permeates the ultrafiltration membrane tosolidify.

[6] A ceramic filter comprising: a porous base member; anultrafiltration membrane formed on the porous base member and having anaverage pore diameter of 2 to 20 nm; and a ceramic porous membrane whichis formed on the ultrafiltration membrane and which partially permeatespores of the ultrafiltration membrane.

[7] The ceramic filter according to the above [6], wherein a permeationdepth of the ceramic porous membrane in the ultrafiltration membrane is½ or more of a thickness of the ultrafiltration membrane from theoutermost surface of the ultrafiltration membrane.

[8] The ceramic filter according to the above [6] or [7], wherein theultrafiltration membrane is a titania membrane.

[9] The ceramic filter according to any one of the above [6] to [8],wherein the ceramic porous membrane is a silica membrane.

[10] The ceramic filter according to any one of the above [6] to [9],wherein the ceramic porous membrane is formed by allowing a ceramic solto permeate the ultrafiltration membrane and solidify.

The thin ceramic porous membrane having less defects can be formed usinga constitution of the ceramic porous membrane which is formed on theultrafiltration membrane having an average pore diameter of 2 to 20 nmand which partially permeates the pores of the ultrafiltration membrane.According to a structure in which a part of the ceramic porous membraneinfiltrates the ultrafiltration membrane, influences of an unevensurface of a porous base member are reduced, and the ceramic porousmembrane having less defects and having a dehydrating function with ahigh resolution can be obtained. Furthermore, when such a structure isused, the ceramic porous membrane having a high performance can beobtained with reduced costs. When the silica membrane is used as theceramic porous membrane, the membrane is especially preferable for anapplication of dehydration of alcohol such as ethanol or isopropylalcohol or an organic acid such as acetic acid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a ceramic filter according to oneembodiment of the present invention;

FIG. 2 is a perspective view showing a ceramic filter according to oneembodiment of the present invention;

FIG. 3 is a schematic diagram schematically showing one example of amethod of manufacturing a silica membrane of the ceramic filteraccording to the present invention;

FIGS. 4(a)(b)(c) are explanatory views of a silica membrane in a casewhere a titania UF membrane is formed;

FIGS. 5(a) to 5(e) are explanatory views of a silica membrane in a casewhere any titania UF membrane is not formed;

FIGS. 6(a)(b)(c) are explanatory views of formation of the silicamembrane at a defective membrane; and

FIG. 7 is a diagram showing a flux with respect to a separationcoefficient.

DESCRIPTION OF THE REFERENCE NUMERALS

-   1: silica membrane, 10: ceramic filter, 11: porous base member, 14:    titania UF membrane, 22: partition wall, 23: cell, 25: inlet-side    end surface, 40: coating liquid (silica sol liquid), 41: masking    tape, 51: foreign matter, 52: hole.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will hereinafter be describedwith reference to the drawings. The present invention is not limited tothe following embodiment, can be changed, modified or improved withoutdeparting from the scope of the present invention.

FIG. 1 shows a silica membrane 1 which is a ceramic porous membrane ofthe present invention. The silica membrane 1 is formed on a titania UFmembrane 14 which is an ultrafiltration membrane (also referred to asthe UF membrane) formed on a porous base member 11 as a microfiltrationmembrane (also referred to as the MF membrane) and having an averagepore diameter smaller than that of the porous base member 11. The silicamembrane has an average pore diameter smaller than that of the titaniaUF membrane 14, and a part of the silica membrane permeates the titaniaUF membrane.

It is preferable that the porous base member 11 is the microfiltrationmembrane (the MF membrane) having pore diameters of about 0.1 to 0.6 μmat an outermost layer.

Moreover, the titania UF membrane 14 which is an ultrafiltrationmembrane having pore diameters of about 2 to 20 nm (preferably about 8nm) is formed on the microfiltration membrane (the MF membrane) 11, andthe silica membrane 1 is formed on the titania UF membrane 14. It isassumed that the silica membrane 1 has an infiltration structure toinfiltrate the titania UF membrane 14. In other words, a part of silicaforming the silica membrane 1 permeates the titania UF membrane 14.Here, when silica permeates the titania UF membrane 14, it is indicatedaccording to EDX element analysis that a portion having a silica/titaniaoxide weight ratio of 0.2 or more has a thickness of ½ or more of thatof the UF membrane from the outermost surface of the UF membrane. It isassumed that the silica/titania oxide weight ratio is an average valueof ten measurements of spot analysis based on the EDX element analysis.

In a case where the silica membrane 1 is formed on the titania UFmembrane 14 having pore diameters of about 2 to 20 nm as describedabove, when a membrane surface of the titania UF membrane 14 is smoothand has less defects, the silica membrane 1 can be formed to be thinwithout any defect. That is, the silica membrane 1 having a highseparability and a high flux (a transmitted and filtered flux) can beprepared with reduced costs.

On the other hand, when the silica membrane 1 is formed on titaniahaving pore diameters of 20 nm or more, owing to the unevenness of thesurface, a silica layer constitutes a thick membrane in order to coverthe whole surface with the silica membrane 1, thereby resulting in a lowflux. Owing to the unevenness of the surface, the silica membrane 1becomes non-uniform, and defects such as cracks are easily generated.That is, a low separation performance is obtained. Furthermore, toprevent the generation of the cracks, an only thin membrane is formedonce. The number of steps increases, and hence the costs increase.

In a case where the titania UF membrane 14 is used as a base for theformation of the silica membrane 1 and the silica membrane 1 is formedon the titania UF membrane 14 to constitute a structure in which silicainfiltrates the titania UF membrane 14, influences of the unevenness ofthe MF membrane are reduced, and the silica membrane 1 having lessdefects, that is, the silica membrane 1 having a high separability canbe formed.

Next, one embodiment of a ceramic filter 10 in which the silica membrane1 is formed according to the present invention will be described withreference to FIG. 2. The ceramic filter 10 of the present invention hasa monolith shape including a plurality of cells 23 defined by partitionwalls 22 to form channel passages in an axial direction. In the presentembodiment, the cells 23 have a circular section, and the silicamembrane 1 shown in FIG. 1 is formed on an inner wall surface of each ofthe cells. The cells 23 may be formed so as to have a hexagonal orquadrangular section. According to such a structure, for example, when amixture (e.g., water and acetic acid) is introduced into the cells 23from an inlet-side end surface 25, one of constituting elements of themixture is separated at the silica membrane 1 formed on an inner wall ofeach cell 23, transmitted through the porous partition walls 22 anddischarged from an outermost wall of the ceramic filter 10, so that themixture can be separated. That is, the silica membrane 1 formed in theceramic filter 10 can be used as a separation membrane, and has a highseparation characteristic with respect to water and acetic acid.

The porous base member 11 which is a base main body is formed as acolumnar monolith-type filter element formed of a porous material byextrusion or the like. As the porous material, for example, alumina canbe used, because this material has a resistance to corrosion, porediameters of a filtering portion scarcely change even with a temperaturechange and a sufficient strength can be obtained. However, instead ofalumina, a ceramic material such as cordierite, mullite or siliconcarbide may be used.

Since the silica membrane 1 of the present invention is formed on aninner peripheral surface (the inner wall surface) of the porous basemember 11, a comparatively long cylindrical base having a length of 50cm or more, or a porous base member having a lotus-root-like shape canpreferably be used.

Moreover, the titania UF membrane 14 is formed on the porous base member11, and the silica membrane 1 is formed on the titania UF membrane 14.That is, an ultrafiltration membrane (the UF membrane) is formed on atleast a silica membrane 1 forming surface of the base formed of theporous material. It is preferable to form, as the ultrafiltrationmembrane, a titania membrane which inhibits generation of particles orpolymers in a range of 0.1 μm to 2 nm. It is assumed that an averagepore diameter of the titania membrane is smaller than that of the porousmaterial.

Next, a method of manufacturing the silica membrane 1 will be describedwith reference to FIG. 3. First, a coating liquid (a silica sol liquid)40 for forming the silica membrane 1 is prepared. To prepare the coatingliquid 40, tetraethoxy silane is hydrolyzed in the presence of nitricacid to form a sol liquid, and the sol liquid is diluted with ethanol.The liquid may be diluted with water instead of ethanol.

Next, as shown in FIG. 3, an outer peripheral surface of the porous basemember 11 provided with the titania UF membrane 14 is sealed with amasking tape 41. The porous base member 11 is fixed to, for example, alower end of a wide-mouthed rotor (not shown), and the coating liquid(the silica sol liquid) 40 is passed through the cells 23 from an upperportion of the base. Alternatively, instead of this process, a membraneformation process by usual dipping may be used. Subsequently, atemperature is raised at a ratio of 100° C./hr, retained at 500° C. forone hour, and then lowered at a ratio of 100° C./hr. Operations such asthe passing of the coating liquid (the silica sol liquid) 40, drying,temperature raising and temperature lowering are repeated three to fivetimes.

According to the above steps, the silica membrane 1 is formed on thetitania UF membrane 14. That is, as shown in FIG. 4(b), the titania UFmembrane 14 is formed on the porous base member 11 shown in FIG. 4(a).In consequence, influences of the unevenness of the surface of theporous base member 11 are reduced by the titania UF membrane 14.Therefore, as shown in FIG. 4(c), the silica membrane can be formed as athin membrane having less defects. That is, the silica membrane 1 havinga high flux and a high separability can be formed with reduced costs.

On the other hand, in a case where the silica membrane 1 is directlyformed on the surface of the porous base member 11 shown in FIG. 5(a),even when a silica membrane 1 a is formed as shown in FIG. 5(b), thewhole surface cannot be covered, and cracks are easily generated in thesilica membrane 1 owing to unevenness. As shown in FIGS. 5(c) to 5(e),when silica membranes 1 b, 1 c and 1 d are superimposed to form a thickmembrane, the silica membrane 1 can be flattened, but in this case, alow flux results. Since the number of the steps increases, the costsincrease.

Characteristics of a case where silica of the silica membrane 1infiltrates the titania UF membrane 14 will be described with referenceto FIGS. 6(a)(b)(c). As shown in FIG. 6(a), in a case where there is aforeign matter 51 on the titania UF membrane 14 and a structure in whichthe silica membrane 1 infiltrates the titania UF membrane 14 as shown inFIG. 6(b) is formed, even when there are foreign matters and defects,the silica membrane 1 infiltrates so as to cover the titania UF membrane14. Even when the titania UF membrane 14 has a hole 52, the silicamembrane 1 is formed so as to cover the hole 52. A structure in whichthe surface of the titania UF membrane 14 is covered with the silicamembrane 1 can be obtained. That is, a foreign matter trace defect and ahole defect can be repaired by the silica membrane 1.

On the other hand, as shown in FIG. 6(c), in a structure in which thesilica membrane 1 does not infiltrate the titania UF membrane 14, whenthe titania UF membrane 14 has the foreign matter defect, thecorresponding portion is not covered with the silica membrane 1. Evenwhen the titania UF membrane 14 has the hole defect, the silica membrane1 is not easily formed at a portion of the hole 52, and a portion wherethe surface of the titania UF membrane 14 is not covered with the silicamembrane 1 is easily generated.

The ceramic filter 10 obtained as described above and including thenano-level thin-membrane-like silica membrane 1 formed on the inner wallsurface thereof can preferably be used as a filter which separates amixed liquid or the like. It is to be noted that when the cells 23 aresubmerged into acetic acid or acetic acid is passed through the cells, aseparation coefficient can be improved. In the above embodiment, thecase where the silica membrane is formed as the ceramic porous membranehas been described, but the present invention is not limited to thisembodiment, and a titania membrane, a zirconia membrane, a zeolitemembrane or the like may be formed.

EXAMPLES

A manufacturing method of the present invention will hereinafter bedescribed in accordance with examples in more detail, but the presentinvention is not limited to these examples. First, a porous base member,a ceramic sol liquid, a membrane forming method and the like used in thepresent example will be described.

Example 1 (1) Porous Base Member

A material provided with an alumina membrane having an average porediameter of 0.2 μm and having a monolith shape (an outer diameter of 30mm, a cell inner diameter 3 mm×37 cells and a length of 500 mm) was usedas a base. It is to be noted that opposite end portions of the base weresealed with glass. The average pore diameter of the base was measuredbased on an air flow process described in ASTM F306.

(2) Titania Sol Liquid

Titanium isopropoxide was hydrolyzed in the presence of nitric acid toobtain a titania sol liquid. A sol particle diameter measured by adynamic optical scattering process was 100 nm.

(3) Titania UF Membrane Formation

The titania sol liquid was diluted with water to obtain a sol liquid formembrane formation. The liquid was circulated through base cells to comein contact with the cells, whereby the membrane was formed in the cells.

(4) Drying, Firing

After a sample was dried, the sample was thermally treated at 500° C.This sample was used as a titania UF base provided with the titania UFmembrane. When pore diameters of the titania UF base were measured, anaverage pore diameter was 8 nm. A measurement principle of the porediameters is the same as that of the method described in Non-PatentDocument 1, but in the Non-Patent Document 1, water vapor and nitrogenwere used, whereas in the measurement method used in the presentinvention, n-hexane and nitrogen were used.

(5) Silica Sol Liquid

Tetraethoxy silane was hydrolyzed in the presence of nitric acid toobtain a silica sol liquid. The silica sol liquid was diluted withethanol, and regulated into 0.7 wt % in terms of silica to prepare a solliquid for membrane formation.

(6) Membrane Formation

An outer peripheral surface of the sample (the porous base member) wassealed with a masking tape. The porous base member was fixed to a lowerend of a wide-mouthed rotor, and 60 ml of silica sol liquid was passedthrough the cells from an upper portion of the base. It was confirmedthat the membrane was formed on the whole inner wall by this membraneformation step.

(7) Drying

The porous base member through which a silica sol was passed was driedat 30° C. for 2 hr on a condition of a humidity of 50%.

(8) Firing

A temperature was raised at a ratio of 100° C./hr, retained at 500° C.for one hour and lowered at the ratio of 100° C./hr. It is to be notedthat the operations of (6) to (8) were repeated three to five times toobtain Example 1.

Comparative Example 1

A titania UF membrane was formed in the same manner as in the membraneformation of Example 1, but a sample was thermally treated at 400° C.This sample was used as a titania UF base provided with the titania UFmembrane. When pore diameters of the titania UF base were measured, anaverage pore diameter was 2 nm. This base was subjected to silica solmembrane formation of (5) to (8).

Comparative Example 2

A titania UF membrane was formed in the same manner as in the membraneformation of Example 1, but a sample was thermally treated at 700° C.This sample was used as a titania UF base provided with the titania UFmembrane. When pore diameters of the titania UF base were measured, anaverage pore diameter was 40 nm. This base was subjected to silica solmembrane formation of (5) to (8).

Comparative Example 3

In the membrane formation of Example 1, any titania UF membrane was notformed, and an alumina porous base member was directly subjected tosilica sol membrane formation of (5) to (8).

In Example 1 and Comparative Example 1, an infiltration depth intotitania UF was measured. In Example 1, a portion in which asilica/titania oxide weight ratio according to EDX element analysis was0.2 or more reached ¾ of a UF thickness from the outermost surface ofthe UF membrane. Therefore, an infiltration structure could beconfirmed. In Comparative Example 1, a portion in which a silica/titaniaoxide weight ratio according to EDX element analysis was 0.2 or morestayed at 1/10 of a UF thickness from the outermost surface of the UFmembrane. Therefore, it could be confirmed that any infiltrationstructure was not obtained.

In Comparative Examples 2, 3, when a membrane was formed using amembrane formation sol liquid having a silica concentration of 0.7 wt %,cracks were generated in the membrane surface, and any membrane couldnot be formed. Therefore, in Comparative Example 2, a membrane wasformed with 0.3 wt %, but cracks were generated. Furthermore, a membranewas formed with 0.1 wt %, the cracks were reduced, and membranes wereformed 20 times. In Comparative Example 3, the cracks were reduced at0.3 wt %, and membranes were formed seven times. However, in eithercase, it was recognized that micro cracks were left in the membranesurface, and any sound membrane could not be formed.

In Example 1 and Comparative Examples 1, 2 and 3, a permeationevaporating separation test was conducted for two hours. In the test,90% ethanol was circulated through monolith cells at 70° C. and a liquidflow rate of 10 L/min, and the outside of monolith was evacuated in arange of 2 to 10 Pa. The sampling was performed four times every 30minutes. As results of the separation tests of Example 1 and ComparativeExamples 1 to 3, a relation between a separation coefficient and a fluxis shown in FIG. 7.

Example 1 indicated a high a (a separation coefficient) as compared withComparative Example 1, and indicated a high a and a high flux ascompared with Comparative Examples 2, 3. In Comparative Examples 2, 3,to form the membrane surface without any crack, a slurry having a smallsilica concentration as compared with the example needs to be used. As aresult, the number of membrane formation times increases. When thenumber of the membrane formation times increases, steps lengthen, andcosts increase.

As described above, when the titania UF membrane is formed on the MFmembrane and the silica membrane is formed on the titania UF membrane, asilica dehydration membrane having a high performance can be obtainedwith reduced costs. The structure in which silica infiltrates titania UFis constituted, and hence a higher separation coefficient can bedeveloped.

According to the present invention, a thin and uniform membrane havingless coarse and large pores and less defects can be obtained with lessmembrane formation times. Therefore, a ceramic filter provided with sucha silica membrane can preferably be used as a filter. A ceramic filterincluding a nano-level thin-membrane-like silica membrane formed on theinner wall surface thereof can be used in a portion where an organicfilter cannot be used, for example, separation removal or the like in anacidic or alkaline solution or an organic solvent.

1. A ceramic porous membrane which is formed on an ultrafiltrationmembrane having an average pore diameter of 2 to 20 nm and whichpartially permeates pores of the ultrafiltration membrane.
 2. Theceramic porous membrane according to claim 1, wherein a permeation depthin the ultrafiltration membrane is ½ or more of a thickness of theultrafiltration membrane from the outermost surface of theultrafiltration membrane.
 3. The ceramic porous membrane according toclaim 1, wherein the ultrafiltration membrane is a titania membrane. 4.The ceramic porous membrane according to claim 1, which is a silicamembrane.
 5. The ceramic porous membrane according to claim 1, wherein aceramic sol permeates the ultrafiltration membrane to solidify.
 6. Aceramic filter comprising: a porous base member; an ultrafiltrationmembrane formed on the porous base member and having an average porediameter of 2 to 20 nm; and a ceramic porous membrane which is formed onthe ultrafiltration membrane and which partially permeates pores of theultrafiltration membrane.
 7. The ceramic filter according to claim 6,wherein a permeation depth of the ceramic porous membrane in theultrafiltration membrane is ½ or more of a thickness of theultrafiltration membrane from the outermost surface of theultrafiltration membrane.
 8. The ceramic filter according to claim 6,wherein the ultrafiltration membrane is a titania membrane.
 9. Theceramic filter according to claim 6, wherein the ceramic porous membraneis a silica membrane.
 10. The ceramic filter according to claim 6,wherein the ceramic porous membrane is formed by allowing a ceramic solto permeate the ultrafiltration membrane and solidify.